Worm gearing Quick question 3

[enfield]

I have in front of me a worm set which is prone to stripping the gears on the worm gear ( machined into the shaft ) It is an oil pump drive from an old Enfield Motorcycle
The shaft is made of a soft steel and its teeth ( worm gear ) appear to be fly cut into the shaft along the axis to the shaft ..ie at 90 deg to the centerline of the worm. With 12 teeth
The pinion is hardened and has a low helix angle 4 deg ... 1 start

Now my question is ..If the contact is sliding wouldnt it be better to have the worm gear teeth at a similar angle to the worm helix ie 4 deg to reduce the point contact, also wouldnt it be better practice to have 2 teeth in contact rather than only 1

I think that as this shaft wears it get to a point where a onetime overload , say when the oil is cold is enough to cause one tooth of the gear to fail

Thanks , this is the first time I have attacked a worm gear in anger , so forgive the questions

Stephen
Ps have read all the other threads on this

 

[Acajun]

Hmmm usually it's the other way around. The worm has the angle and the wheel teeth are at 90 degrees. It's this relationship that causes the sliding motion when the rotating worm pushes the wheel. Think of a jar lid. As for only one tooth in contact with the worm, well that's a first for me, the sets I see usually see have 2 teeth in contact.

Jp

 

[diamondjim]

How many miles are on the motorcycle that
you would break the worm? Seems unusual
as it should be lightly loaded but would
see many cycles. The contact ratio would
have to be greater than one or it would
be jerking all over the place. Was this
a rebuilt pump? I assume the worm is hour
glass shaped and has a lead on it like
a heavy screw thread. The angle of the
helix would match the lead angle of the
screw thread or worm thread. The contact
ratio could be less than two but greater
than one.

 

[enfield]

Thanks for the replies
Sorry for the delay the computer has gremilins inside ..and was not loading drivers ,,
The set up of this bike is ,,,the Shaft is the worm gear ( sorry for the earlier post I didnt see the missing word )
and the worm is hardened single start with a lead of 4 deg and a pressure angle of 20 ( as near as I can measure it )
Having a good long look at this arrangement I cant decide if the the teeth have adequate contact , IT looks like Worm tooth ...worm gear tooth ,,,worm tooth
But with the tolerances this thing has been made to I cannot say with certainty
Also the worm is straight so in point contact ie not shaped like an hour glass
and the shaft runs on plain bearings

The teeth on the worm gear are failing by sliding wear after about 15 to 20 000 km ,

My plan would be to change the material of the worm gear from steel ( not sure what yet ) and to increase the helix angle of the worm gear from 0 to equal the lead angle of the worm ...as well as try to use ball bearings to acuratly locate the shaft in relation to the worm

Thanks for you valuable imput

Stephen

 

[powertrans]

The life of your gearset may be influenced by:
1. Materials and hardness
2. Geometry of threads and teeth
3. Lubrication
4. Stiffness and alignment of the mounting
5. Loads and cycles

The best material combination would probably be as hard a steel as you can manage for the worm and a medium hardness bronze for the gear. Oil or grease lubrication must be present at the mesh at all times. I agree with earlier posts that the loads are likely light on your gears, so if you get the materials, geometry and lubrication right, they should last much longer. Good luck!

 

[enfield]

Hi there

I have no choice in the loads, Starting on them soon
The Geometry in the original set is all over the show( I think )
Lubrication it sits in an oil bath I will have the material specs tonite so can make a choice from there

Also Would it be better the helix angle of the worm gear ( at the moment its 90deg to the axis of the worm ) If I reduce the helix to 86 deg so that the lead of the worm is almost the same as the the helix of the gear
I suspect this would reduce the sliding force ( I havent started on this so not sure what force does what) but would increase the separating force ?
So Yes I am lurking around Geometry and hardness of the worm gear as a change.
As well as accurate location ( these things run in a plain bearing direct in Aluminium housing ,,,,the saving grace they are an oil pump so there is a bit of oil about ,,)

Thank you for the kind replys its difficult if you dont often ( never ) design worm gears then get thrown in the deep end ...and $ How much amazon for the book.
The replies are a big help even if they just confirm, it help s you feel like you are on the right track rather than floundering in the deep end

Stephen

 

[ShadowManager]

Although one would have to physically see the mechanics of your Worm problem to provide a valid opinion. Have you concidered adding a secondary Gear to act as an idler over top of the Worm? Providing enough space, "Sandwitch" the Worm in between two gears to eliminate the mountain effect of the worm climbing out of the worm. Lubrication is very important especially in worm applications.

Terry A Hrushka Ph.D., M.S.Eng., B.Sc.